A gasoline-ethanol blend fuel has been studied recently for use as an automobile fuel. By using bioethanol to be produced by fermentation and distillation of vegetable substances as the ethanol, it is believed that, if soil management is carried out strictly, a so-called carbon neutral effect can be attained to decrease the amount of carbon dioxide emission and to contribute to prevention of global warming.
However, a problem is that if farm products, such as sugarcane and maize, are used as the vegetable substances, a large amount of such farm products are consumed as a source material for ethanol, and supply of food or feed is reduced. Consequently, a technology for producing ethanol using lignocellulose-based biomass not suitable for food or feed as the vegetable substances has been studied.
Since the lignocellulose-based biomass contains cellulose, ethanol can be yielded by degrading the cellulose to glucose by a saccharification treatment using a saccharifying enzyme and fermenting the yielded glucose. As such a saccharifying enzyme, for example, a cellulase originated from Acremonium cellulolyticus has been heretofore known (see e.g. Patent Literature 1).
Since the saccharifying enzyme is expensive, the concentration of lignocellulose-based biomass as a substrate is set at a low level during the ethanol production in order to reduce the usage of the saccharifying enzyme. However, if the concentration of the substrate is set at a low level, the concentration of a yielded saccharified solution becomes also low, and therefore the concentration of ethanol to be yielded by fermenting the saccharified solution becomes also low. As the result, there occurs a problem that the time and thermal energy required for distillation increase, when the yielded ethanol is distilled to be concentrated.
To solve the problem, it is conceivable to increase the concentration of the substrate as well as the usage of the saccharifying enzyme to a high level, so as to yield a high concentration of ethanol, and to reduce the energy required for concentration, distillation and the like of ethanol, such that the overall energy efficiency is improved. In this case, however, since the saccharifying enzyme is expensive, a problem is that the cost increases due to the increase in the usage.
Consequently, to solve the problem, it is desired that the saccharification efficiency is maintained and at the same time the usage of a saccharifying enzyme is reduced.